Flash memories have advanced performances in accessing data, than any other kind of nonvolatile memories such as electrically erasable and programmable read only memories, for a reading and writing (or programming). The merit of high speed operation in the flash memory has been regarded to be very adaptable to portable computing apparatuses, cellular phones or digital still cameras. In general, there are two kinds of the flash memory, such as the NAND-type in which memory cells are connected from a bit line in serial, and the NOR-type in which memory cells are connected to a bit line in parallel.
Typical construction of the cell (or cell transistor) of the flash memory is shown in FIG. 1, which can be used for the multi-bit storage. Source 3 and drain 4, each being formed of N+ diffused region in P+ semiconductor substrate 2, are separated each other through a channel region which is also defined in substrate 2. Floating gate 6 is formed over the channel region through thin insulating film 7 which is under 100 .ANG., and insulating film 9, such as an O-N-O (Oxide-Nitride-Oxide) film, on floating gate 6 isolates control gate 8 from floating gate 6. Source 3, drain 4, control gate 8 and substrate 2 are each connected to their corresponding voltage sources Vs (drain voltage), Vd (source voltage), Vg (gate voltage) and Vb (bulk voltage), for programming, erasing and reading operations.
In programming, as well known, a selected memory cell is programmed by means of a hot electron injection between the channel region and floating gate, in which the source and substrate are held in a ground voltage, a high voltage (e.g., Vg=10 V) is applied to the control gate and a voltage to induce the hot electrons therein, 5 V through 6 V, is provided to the drain. After programmed, a threshold voltage of the selected memory cell is increased therefrom due to deposition of electrons. To read data from the programmed cell, a voltage of about 1 V is applied to the drain, a power source voltage (or about 4.5 V) is applied to the control gate, and the source is held in the ground voltage. Since the increased threshold voltage of the programmed memory cell acts as an blocking potential even upon the gate voltage during a read-out operation, the programmed cell is considered to as an off-cell which has a threshold voltage between 6 V and 7 V.
Erasing a memory cell is accomplished by conducting F-N (Fowler-Nordheim) tunneling effect, in which the control gate is coupled to a high negative voltage of about -10 V, and the substrate (or bulk) to a positive voltage of about 5 V, in order to induce the tunneling therebetween. While this, the drain is conditioned at a high impedance state (or a floating state). A strong electric field induced by the voltage bias conditions, between the control gate and bulk region, causes the electrons to be moved into the source. The F-N tunneling normally occurs when the electric field of 6.about.7 MV/cm is developed between the floating gate and bulk region which are separated through the thin insulating film under 100 .ANG.. The erased cell has a lower threshold voltage than before, and thereby sensed as an on-cell which has a threshold voltage between 1.about.3 V.
In an usual architecture of a memory cell array in a flash memory, the bulk region (or the substrate) combines active regions of memory cells, so that memory cells formed in the same bulk region are spontaneously erased in the same time. Therefore, units of erasing (hereinafter referred to as "sector", for instance, one sector of 64K) is determined in accordance with the number of separating the bulk regions. Table 1 shows levels of the voltages used in programming, erasing and reading.
TABLE 1 ______________________________________ operation mode Vg Vd Vs Vb ______________________________________ programming 10V 5.about.6V 0V 0V erasing -10V floating floating 5V reading 4.5V 1V 0V 0V erase repairing 3V 5.about.6V 0V 0V ______________________________________
Once an erase operation for memory cells is performed, there are memory cells, having their threshold voltages under 0 V, among memory cells which are erased and whose threshold voltages are positioned at an range of distribution uniformity. The memory cells with threshold voltage of 0 V are referred to as over-erased cells that have to be cured by an repairing operation (i.e., erase repairing) in order to have threshold voltages properly higher than 0 V. In a general erase repairing operation, the source and substrate of the over-erased memory cell are grounded, the control gate is connected to a positive voltage about 3 V that is lower than a program voltage (e.g., 10 V ), and the drain is connected to a positive voltage about 5 V through 6 V, so that negative charges smaller than those in the programming operation are gethered in the floating gate electrode and, as shown in FIG. 2, thereby the negative potential formed at the gate electrode causes the threshold voltage of the memory cell to be increased over 0 V (or the ground voltage). The erase repairing operation is performed after completing a normal erasing and referred to as post-programming.
The post-programming operation, as shown in FIG. 3, is subsequent to a main erase operation which is carried out just after pre-programming. The pre-programming, the main erasing and post-programming compose a general erase cycle. The pre-programming employs a biasing condition that is the same with that of a normal programming, in which selected word line and bit line are conductive with 10 V and 5 V, respectively, so as to prevent an occurrence of an over-erased cells in the main erasing subsequent thereto.
Threshold voltages of pre-programmed memory cells moves to the levels at least higher than about 6 V (i.e., a threshold voltage of an Off-cell). Next, the main erase operation is performed to make memory cells be on-cells. Since there could be over-erased cells after the main erasing although the pre-programming has been preceded, the post-programming step is activatedjust after completing the main erasing operation. In each of the steps of the pre-programming, the main erasing and post-programming, verifying steps are interstitially included therein to determine whether or not further programming or erasing steps are in need. While those operating steps for erasing and curing, an usual flash memory could receive specific commands such as "suspend" and "resume". The command "suspend" causes a present erase operation for a selected sector to be interrupted therein, but rendering other sectors to be operable of reading and programming, and the command "resume" makes the halted erase operation be activated again.
FIG. 4 shows a operating timing when the suspending command is applied to a flash memory while the aforementioned erase mode (including the pre-programming, the main erasing and post-programming) is being conductive. For more detail, see U.S. Pat. No. 5,355,464 entitled Circuitry and Method for Suspending the Automated Erasure of Non-Volatile Semiconductor Memory. As shown in FIG. 4, if the flag signal for suspending, F.sub.-- suspend, is activated during the period of main erasing, reading and programming for another sector are performed after completing the current erasing operation, i.e., after e main erasing flag signal F.sub.-- mainera is disabled to low). It is impossible to be operable of reading or programming for another sector until the main erasing for a current sector is over. The period from the input of the suspend command to the time expiring the current main erasing, Tx, does not permit any other operation but the currently-conductive main erasing operation. The read or programming for another sector is permitted to be conductive for the time period Ta for which busy signal BUSY is being inactivated after the main erasing is over. In the state of inactivating with busy signal BUSY after the main erasing, if the flag signal of resuming F.sub.-- resume is applied thereto, the next step of the main erasing, i.e., post-programming, starts to proceed for the sector selected in the main erasing operation.
However, in the operation shown in FIG. 4, since any functional operation can not be enabled but the current main erasing when the command "suspend" appears therein, there would be an limit in extending an operational application that, for instance, it is necessary to quickly turn a current operation to a read or programming for another sector after the command of suspending.